Lillian Luthor is a scientist, the leader of Project Cadmus and the biological mother of Lex Luthor and adoptive mother of Lena Luthor. She serves as the secondary antagonist of the second season of the CW TV series Supergirl, a minor antagonist of the third season and a minor character of the fourth season.
Lillian met her future husband Lionel Luthor while working as a physician. Some time after they married, they had a son, whom they named Lex Luthor. During this time, however, Lionel began to have an affair with an unknown woman; Lionel, seeking to patch things up with Lillian, paid the woman, pregnant at the time, to keep quiet about their relationship. The woman eventually gave birth to Lionel's second child, Lena.
When Lena was four years old, her mother died and Lionel decided to bring Lena to live with them. Lillian begrudgingly agreed to this and she and Lionel adopted Lena, never revealing that she was actually Lionel's biological daughter.
Lillian is shown to be very much like her son, Lex; cold, self-righteous, unstable and delusional in her views. Blaming Superman for her son's imprisonment, while completely ignoring the horrific actions Lex performed, she has come to develop radically-driven disdain for aliens of all kinds. Viewing them as a poison to Earth, beings who are subtly taking control of the planet by making humans worship them, she is determined to purge Earth of these otherworldly-forms. To that end, she takes over Project Cadmus, a secret lab that studies aliens, and severed its relationship with the government.
She is shown to be rather hypocritical, willing to kill bystanders, the very people who she claims to trying to save, in her personal vendetta. While arrogant in her views and believing herself the true savior of the planet, Lillian is apparently a very shrewd and cunning woman. Able to anticipate how her enemies will act, she is able to plan several steps ahead and manipulate their actions.
However, Lillian's arrogance makes her short-sighted (as she doesn't realize when someone close to her is manipulating or deceiving her); when her adoptive daughter, Lena apparently joined forces with her to release a deadly virus that would wipe out all alien life on earth. However, Lena had no intention of releasing the virus; she switched a critical ingredient to make the virus inert, and alerted the police about her mother's crimes. Lillian only realized she'd been double-crossed when the missile carrying the virus exploded and only sent red rain falling on National City. Before she could react, the police swarmed in and arrested Lillian for attempted mass murder and other Cadmus-related crimes.
- In the DC comics, Lex Luthor's mother is named Arlene Luthor. She debuted in Superman's Girlfriend, Lois Lane #23 (February, 1961) and was created by Jerry Siegel and Kurt Schaffenberger. In later incarnations her name was changed to Letitia, even though in most of her appearances she remains unnamed.
- The character was first given the name "Lillian" on the TV show, Smallville, on which she made her first live-action appearance.
- Despite her behavior toward Lena, it is revealed in the penultimate episode of season four that Lillian does, in fact, love her, though it took having a baby Truth-Seeker alien attached to her arm to get her to admit that.
- At the end of season four, Lena finds out that Kara is Supergirl. Though the writers' plans are currently unknown, she might turn into a bigger villain than her mother.